Both magnetic resonance tomography (MRT) and positron emission tomography (PET) are imaging methods that are suitable for displaying the interior of the human body and, in particular PET, for displaying biochemical processes. There has recently been an interest in combining MRT and PET with one another in a unit in order to be able to apply both imaging modalities simultaneously or shortly after one another to the same patient.
It is proposed to this end to additionally provide a possibly removable PET detector inside the magnet arrangement of basic field magnet and gradient coils customary in a magnetic resonance apparatus. A radio frequency coil arrangement is to be arranged coaxially for the MRT images inside this PET detector arrangement, which is mostly surrounded by a radio frequency shield.
The coordinate system in which the pictures are produced during magnetic resonance imaging is determined by the gradient system of the magnetic resonance apparatus. It is possible in this case to superpose on the flux generating gradient currents an offset that linearly displaces the coordinate system. In addition, modern magnetic resonance apparatuses mostly comprise so-called shim coils or other shim device(s) that superpose on the static magnetic field a small regulatable magnetic field, and thus specifically compensate distorting influences of interference fields of the magnetic resonance apparatus itself or of the environment. The coordinate system of the magnetic resonance scanner is additionally displaced by this additional magnetic field.
To calibrate the scanner or to undertake a later checking of the coordinate system for the magnetic resonance imaging, use is made of so-called magnetic resonance phantoms that include visible markers at predetermined points in the magnetic resonance imaging and are, for example, filled with a substance that forms a visible marker in the magnetic resonance imaging, in particular water. It is also possible to provide an extended marker that is provided in a specific form and with the aid of which calibration is possible. The markers are consequently filled with materials whose magnetic resonance echo time is preferably close to that of biological tissue.
In order to be able to intercompare images of MRT and PET, it is necessary to know the transformation between the measurement coordinate systems of the two imaging systems. There is likewise a need to check this transformation regularly, for example when shim currents are being adapted or the offset is displaced in another way. It is problematic here that the markers of the magnetic resonance phantoms are not visible in the PET imaging, that is to say no PET signal is generated.
There is thus a need for a device and a method that enable the calibration of a magnetic resonance apparatus having a PET function, particularly with regard to the relationship between the measurement coordinate systems of the PET and the MRT.